Three objectives define this total population study of behavior problems in preschool children. The first objective is to determine the prevalence of behavior problems in 3-year-old children; the second, to examine the stability and change of behavior problems from the preschool age to school age; and the third, to investigate the origins of behavior problems, in children under the age of 3 years. The sample is drawn from a register of families with preschool children in permanent residence on Martha's Vineyard Island, Massachusetts. A prevalence study, conducted in 1978-79, selected 14% of the total population of 3-year-olds to have a behavior problem. Initial emphasis was placed on the validity of the methods used to identify children with behavior problems. Currently, a variety of home environment and temperamental characteristics of the children are being correlated with behavior problems. A follow-up of these same children is scheduled for 1981-83, as they complete first grade. At the same time a longitudinal study beginning in infancy is beginning to investigate the roles of family stress and infant temperament in the origins of behavior problems.